This invention relates generally to cathode ray picture tubes, and is specifically addressed to an electron gun for use in very small picture tubes intended primarily for dashboard installation in vehicles. The invention also has utility in other very small cathode ray picture tube applications.
An electron gun for use in a cathode ray tube intended for a vehicle (a "VCRT"), must meet severe operational and physical constraints. These constraints are best pointed up by comparison of the VCRT with the standard cathode ray tube of five-inch diagonal measure intended for a small consumer product television receiver.
Peak brightness of the VCRT, for example, is preferably of the order of ten times the brightness (under typical operating conditions) of a cathode ray tube used in a consumer product television receiver. This magnitude of brightness is mandatory in compensating for the brightness loss caused by the neutral density filtering normally used to increase contrast. The electron gun beam current and beam energy is not the sole factor in providing the necessary brightness; special, high-performance phosphors are also required.
Because of space constraints, especially in behind-the-dashboard installations, the VCRT, as a practical matter, must be ultra-short; that is, preferably less than five inches in depth (or length), and must have a very wide deflection angle, preferably greater than 110 degrees. In contrast, the standard CRT with a five-inch face panel is normally about eight inches in length, and the deflection angle is commonly a relatively narrow 70 degrees.
The length of the electron gun must be greatly reduced if the objective of a very short VCRT is to be fulfilled. The reducing of gun length presents major problems to the gun designer that resulted in the inventive solution set forth herein.
Quick comprehension of a displayed message for quick decision making is a firm requirement in VCRT's, so the resolution of the tube must be high. The degree of resolution is largely a function of beam spot size in monochrome cathode ray tubes. The spot size of a VCRT of less than five inch length is preferably no greater than about 40 mils at 2,000 foot-Lamberts, for example, while the spot size of the standard five-inch CRT cited for comparison provides satisfactory performance with a spot size of about 80 mils at the same brightness.
With regard to power consumption, the large deflection angles and higher beam currents results in a higher power consumption--of the order of three times that required by the standard five-inch CRT.
In a journal article titled "Theoretical and Practical Aspects of Electron Gun Design for Color Picture Tubes," Dr. I. M. Wilson provides some insight into the theoretical and practical constraints imposed on the designer of electron guns for color picture tubes. (IEEE Transactions on Consumer Electronics, Volume CE-21, No. 1, February 1975.) The analytical method described takes into account the three principal effects that determine the size of the focused beam on the screen--magnified cross-over, spherical aberration of the lens, and space charge in the drift space. The calculated and measured data for a gun using this design method is presented. The relevance of this disclosure to the present invention is set forth infra.
A high-voltage bipotential gun is described in a journal article by R. H. Hughes and H. Y. Chen of the RCA picture tube division. It is alleged that its performance is significantly improved over the conventional bipotential gun and is approximately equivalent to the tripotential gun. The relevance of this disclosure to the present invention is also set forth infra. (Hughes, Chen: "A Novel High-Voltage Bipotential CRT Gun Design," 0098-3063/79/0400-0185, IEEE.)
In UK Patent Application GB No. 2 020 092 A (RCA), there is disclosed a high potential, low magnification electron gun for use in color picture tubes. This bipotential-type gun comprises a cathode, an apertured plate control grid (G1), an apertured plate screen grid (G2), and at least two tubular focusing electrodes (G3 and G4). It is stated that the quality of the gun's beam spot may be improved by (1) establishing an operating electric field between the G2 and G3 which is between about 100 and 400 volts/mil thereby reducing aberration effects in the beam-forming region of the gun; (2) making G2 thick so as to prevent the high G3 voltage from penetrating the region between G1 and G2, thereby allowing the G1-G2 field to provide a divergent effect on the electron beam prior to beam crossover and thus give a reduced crossover angle; (3) elongating G3 to provide an optimum filling of the main focus lens with a beam to maximize the object distance of the focusing system; and (4) structuring G2 and G3 to provide a flat electrostatic field therebetween to avoid prefocusing action in that region so as not to cause an effective reduction of the object distance of the focusing system. The focusing electrode (G3) potential is specified as being 8.5 kV and the accelerating anode (G4) potential is 30 kV; this gun merits the appellation "HiBi" because of the relatively high focusing voltage. ("HiBi" is an abbreviated acronym for "high focus voltage bipotential (gun).")